Processes for preparing coated catalysts of the aforementioned type are known, for example from WO 95/11081, WO 2004/108267, WO 2004/108284, US-A 2006/0205978, EP-A 714700 and DE-A 102005010645. The active composition is a multimetal oxide comprising molybdenum. The term “multimetal oxide” expresses the fact that the active composition, as well as molybdenum and oxygen, also comprises at least one further chemical element.
Catalysts of the aforementioned type are suitable, for example, for the catalysis of the heterogeneously catalyzed partial gas phase oxidation of acrolein to acrylic acid.
DE-A 10350822 and DE-A 102004025445 disclose that the heterogeneously catalyzed gas phase partial oxidation of acrolein to acrylic acid over one and the same fixed catalyst bed can be operated essentially continuously over prolonged periods. However, the activity of the fixed catalyst bed worsens in the course of operation.
In order nevertheless to be able to operate the fixed catalyst bed, whose exchange is comparatively inconvenient and costly, for as long as possible, attempts are made in the prior art in a wide variety of ways to counteract the aging process of the fixed catalyst bed.
EP-A 990636 and EP-A 1106598 propose substantially compensating for the reduction in the activity of the fixed catalyst bed by, in the course of the operating time, under otherwise substantially constant operating conditions, gradually increasing the temperature of the fixed catalyst bed, in order to essentially maintain the acrolein conversion in single pass of the reaction gas mixture through the fixed catalyst bed. A disadvantage of this procedure is that, with increasing elevation of the temperature of the fixed catalyst bed, the aging process is increasingly accelerated. Finally, the catalyst bed has to be exchanged in its entirety.
DE-A 102004025445 proposes, for the long-term operation of the heterogeneously catalyzed partial gas phase oxidation of acrolein to acrylic acid, counteracting the deactivation of the fixed catalyst bed by increasingly elevating the working pressure in the gas phase with increasing operating time of the fixed catalyst bed. A disadvantage of this procedure is that, with increasing working pressure in the heterogeneously catalyzed partial gas phase oxidation, elevated compression outputs are required.
EP-A 614872 recommends prolonging the lifetime of the fixed catalyst bed by, after an operating time of the fixed catalyst bed of several years, interrupting the process for partial oxidation and, at elevated temperature, conducting a regeneration gas mixture composed of oxygen, steam and inert gas through the fixed catalyst bed, and then continuing the partial oxidation.
What is common to the above-described prior art processes for prolonging the lifetime of the fixed catalyst bed is that they do not preventatively counteract the deactivation of the fixed catalyst bed, but rather attempt to counteract the consequences of the deactivation which has already occurred.
EP-A 0 630 879 describes a process for catalytic oxidation of propene, isobutene or tert-butanol over a multimetal oxide catalyst comprising molybdenum, bismuth and iron, which works in the presence of a molybdenum oxide which is essentially catalytically inactive. The presence of the molybdenum oxide inhibits the deactivation of the multimetal oxide catalyst. The molybdenum oxide may be present in the form of separate molybdenum oxide particles, if appropriate on a support, in a mixture with particles of the multimetal oxide catalyst. Also mentioned is the possibility of preparing a mixture of pulverulent molybdenum oxide and pulverulent multimetal oxide catalyst, and of extruding the mixture to form shaped catalyst bodies or of applying it to a support.
German patent application DE 10 2007 010 422, which was yet to be published at the priority date of the present application, describes counteracting a deactivation of coated catalysts for the heterogeneously catalyzed partial gas phase oxidation of acrolein to acrylic acid, whose active composition is a finely divided multielement oxide which comprises Mo and V and has been applied to a support body, by adding to the catalytically active composition composed of the multimetal oxide comprising Mo and V an oxide of molybdenum or a compound of molybdenum from which an oxide of molybdenum forms. The coated catalyst is coated with a mixture of molybdenum oxide or of the precursor compound and the multimetal oxide.